1. Field of the Invention
This invention relates to tap-changers for voltage regulators and load tap-changing transformers. More particularly, this invention relates to microcontroller-based tap-changer controllers employing half-wave digitization of A.C. signals.
2. Description of the Background Art
In electrical power distribution systems, voltage levels tend to vary due to several factors such as load, line inductance, or line resistance. In order to maintain the voltage level within a predefined range or bandwidth of a fixed voltage level (e.g., 120 volts), load tap-changing (LTC) transformers or series regulating auto transformers using tap-changer switching are employed to incrementally increase or decrease the line voltage.
Typically, tapped auto transformers comprise a tapped series winding that facilitates plus or minus ten percent regulation, a shunt winding across the regulator input terminals, a voltage transformer which measures the output voltage, and a current transformer which measures the load current at the output terminal. A two-position switch is provided which can be placed in a raise or lower position, depending upon whether the regulator is used to "boost" (increase) or "buck" (decrease) the load voltage. The reversing switch is connected across the ends of the series winding. Under this arrangement with the reversing switch in the raise position, the series winding becomes additive with respect to the shunt winding as the number of turns placed in series with the load increases. Therefore, the amount of voltage boost increases. When the reversing switch is moved to the lower position, the series winding, therefore, becomes subtractive with respect to the shunt winding and the amount of the buck depends upon the number of turns placed in series with the line.
The typical load tap-changing transformer and tap-changer switch provide approximately plus or minus ten percent voltage regulation by selecting the proper tap on the transformer secondary. The taps are usually part of a fixed secondary winding and select voltages that are plus or minus a fixed percentage from a nominal voltage.
Presently, there exists many types of automatic tap-changer controls for changing the tap settings of the load tap-changing transformers and regulators. Historically, tap-changers employed analog controllers such as those illustrated in U.S. Pat. Nos. 2,280,766, 2,009,383, and 2,381,271. The more dominant analog tap-changer controls are sold under the registered trademarks "Siemens (Allis)" Models MJ-1A, MJ-2A, MJ-3, MJ-3A, IJ-2, IJ-2A, SJ-4, SJ-5, SJ-6, UA and UJ, "General Electric" Model ML-32, VR-1, SM-2A, "Cooper" Model CL-2, CL-2A, CL-4A, CL-4B and CL-4C and "Beckwith" Models M-0067 and M-0270 series.
More recently, microprocessor-based tap-changer controllers have been developed such as the one disclosed in U.S. Pat. No. 4,419,619 issued to McGraw-Edison Company (now Cooper Power Systems). In this McGraw-Edison tap-changer controller, the microcomputer is interfaced to the regulator by means of interface circuits that provide digital data of sampled voltage and current signals to the microcomputer. Software employed within the microcomputer performs Fast Fourier Transforms (FFT) on the sampled voltage and current signals. The McGraw-Edison tap-changer controller uses an external data acquisition system which includes a bi-polar analog to digital (A/D) converter with the associated circuitry of a multiplexer, a sample and hold circuit and a bi-polar voltage reference. In addition to the external data acquisition system, external circuits also include programmable timers, serial communications interfaces, reprogrammable non-volatile memory and peripheral interface adapters. This McGraw-Edison controller has a second voltage input which measures the voltage on the "difference" winding across the source to load of the regulator that supplied voltage difference information to the control. Using this information, the controller calculates the tap-changer position, as it knows the voltage differential per tap of the regulator. Also, the difference voltage is used to calculate the source voltage for regulation during reverse power operation. However, this method requires an additional analog voltage input signal and would only be applicable to regulators equipped with a voltage differential winding.
A microcomputer-based tap-changer controller provides many advantages over analog tap-changer controllers, such as accuracy, flexibility, ease of use and adaptability. Microcomputer-based tap-changer controllers may be connected to a central computer via a serial communications port to achieve more automated power distribution.
There presently exists a need for a microcontroller-based tap-changer controller that employs an accurate yet simpler data acquisition system and simplified external hardware to the microcontroller along with methods for calculating the source side voltage for reverse power operation without the need for a second voltage input.
Therefore, it is an objective of this invention to provide an improvement which overcomes the aforementioned inadequacies of the prior art controllers and provides an improvement which is a significant contribution to the advancement of the tap-changer controller art.
Another objective of this invention is to provide a microcontroller-based tap-changer controller that accurately controls a conventional tap-changer and yet comprises a simpler and less expensive design than is presently available in microcontroller-based tap-changer controllers.
A further objective of this invention is to provide a single control that can be used interchangeably with a variety of regulators and LTC transformers.
The foregoing has outlined some of the pertinent objectives of the invention. These objectives should be construed to be merely illustrative of some of the more prominent features and applications of the intended invention. Many other beneficial results can be attained by applying the disclosed invention in a different manner or modifying the invention within the scope of the disclosure. Accordingly, other objectives and a fuller understanding of the invention are set forth in the detailed description of the preferred embodiment in addition to the scope of the invention as defined by the claims and taken in conjunction with the accompanying drawings.